It can often happen, for example as the result of manufacturing tolerances in the production of television camera tubes and particularly their electron beam producing systems, that the undeflected electron beam drops centrally incident on the signal electrode of the camera tube. In consequence, the scanning pattern by which the video signal is generated will not be precisely centered on the signal electrode. For correction of this directional error of the electron beam, it is usual to produce mutually perpendicularly directed magnetic fields by means of so-called beam alignment coils disposed around the beam generation system. In order to obtain centering of the electron beam, the direct currents in the alignent coils must be correspondingly adjusted. An important requirement of such alignment is a measuring process for quantitative determination of the directional (alignment) error. Adjustment procedures heretofore used require evaluation of a test picture and one therefore of only most limited suitability for automating the alignment of television cameras.
There is known from German Published Patent Application (AS) No. 20 39 915 and U.S. Pat. No. 3,710,018 a television camera with a camera tube that has a beam alignment system as well as a focussing system such that the effect of the alignment error can be determined by changing the beam focussing field and right afterwards then the alignment coil field until the effects of these fields have produced an alignment error minimum. The field variation and following adjustment steps for precise axial alignment of the scanning beam is in this case automatically carried out. This known system, however, always requires a test picture and, moreover, is relatively expensive and complicated.
There is also known from European Patent No. 103,451 a method for alignment of the electron beam in television camera tubes in which the bias voltage of the signal electrode is so greatly reduced that a measurable shading effect takes place in the camera image. Then for axial alignment of the electron beam the amount of energy supplied to the alignment coils is varied until a measurable amount of such shading takes place in the camera tube. The energy for axial alignment of the electron beam supplied to the alignment coils is then changed until a minimum average shading is obtained. This known method has the disadvantage that complicated and expensive circuit operations are necessary for variation of the signal electrode voltage which, moreover, has a negative effect on the stability or immunity from disturbance on account of the additional parasitic capacitances.